<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <script src="js/vue.js"></script>
</head>
<body>
    <div id="app">
        <div v-if="show">
            用户名: <input type="text" key="user" :placeholder="message">
        </div>
        <div v-else>
            邮箱: <input type="text" key="message">
        </div>
    </div>
    <script>
        var vm = new Vue({
            el:"#app",
            data: {
                show: true,
                message: "hello"
            }
        })
    </script>
</body>
</html>